Recently, the mobile communication including cellular phones provides versatile services in data communication, such as communications in audio, text and dynamic picture. This market trend requires a more sophisticated wireless device, in particular, a wireless device having a more sophisticated antenna is demanded. Because the antenna is a gate for receiving and transmitting an electromagnetic wave, and its performance is one of large number of factors affecting the performance of the wireless device.
A conventional wireless device is described hereinafter with reference to FIG. 3 which illustrates schematically the conventional wireless device having an antenna. In FIG. 3, antenna 103 is placed side by side with ground plane 108. Antenna 103 includes antenna element 101 for resonating with a first frequency and antenna element 102 for resonating with a second frequency. Antenna 103 is coupled to feeding point 104 disposed on ground plane 108, and coupled to radio circuit 107 via matching circuit 105 and transmission line 106. The structure discussed above forms wireless device 109.
As discussed above, the construction of conventional wireless device 109 allows a single feeding point 104 to feed both of first antenna element 101 and second antenna element 102 with electricity. First antenna element 101 resonates with the frequency ranging from 880 MHz–960 MHz, namely, GSM (Global System of Mobile Communication) band, and second antenna element 102 resonates with the frequency ranging from 1710 MHz–1880 MHz, namely, DCS (Digital Communication System) band.
When the wireless device discussed above receives a frequency of GSM band, first antenna element 101 energizes an electric current using an electromagnetic wave received, and the current runs to radio circuit 107 via feeding point 104, matching circuit 105 and transmission line 106. As a result, the electromagnetic wave is received by the wireless device.
When a frequency of GSM band is transmitted from the wireless device, a signal generated in radio circuit 107 is conveyed to first antenna element 101 via transmission line 106, matching circuit 105 and feeding point 104. First antenna element 101 energizes the signal into an electromagnetic wave, which is then radiated, thereby carrying out a transmission.
When the wireless device receives/transmits a frequency of DCS band, second antenna element 102 receives/transmits an electromagnetic wave in the same manner as the case of receiving/transmitting an electromagnetic wave of GSM band.
As such, conventional wireless device 109 deals with the two kinds of frequencies, i.e., GSM and DCS. Japanese patent application non-examined publication No. 2003-101335 discloses one of the prior art related to what is discussed above.
However, since the construction of the conventional wireless device allows one single feeding point 104 to feed both of antenna elements 101 and 102 with electricity, the coupling between elements 101 and 102 is strengthened. Therefore, when an electromagnetic wave is radiated from one antenna, the power radiated travels to the other antenna, so that the one antenna tends to invite some loss in its radiating power.
Further, matching circuit 105 adjusts two different electromagnetic waves independently by itself in order to obtain two different and desirable resonant frequencies, so that when a first resonance frequency is adjusted, a second one changes synchronously. As a result, it is difficult to adjust only the first resonance frequency efficiently and independently of the second one.